1. Field of the Invention
The present invention relates to a wholly aromatic polyamic acid solution composition. More particularly, the present invention relates to a solution composition of a wholly aromatic polyamic acid in an organic polar solvent, useful as an electrically insulating varnish.
2. Description of the Prior Art
It is known that aromatic polyimides which have been prepared by imidizing the corresponding aromatic polyamic acids exhibit excellent heat resistance, electrically insulating property, and mechanical properties. Accordingly, for the purpose of coating various materials, especially, the coating and insulating of various electric materials, for example, electric wires and cords, a solution of an aromatic polyamic acid is applied onto a surface of the material to be coated and then heat-imidized to react with the corresponding aromatic polyimide.
The aromatic polyamic acid is prepared by condense-polymerizing an acid component consisting of at least one tetracarboxylic acid compound with an amine component consisting of at least one diamine compound.
Usually, the tetracarboxylic acid compound is selected from pyromellitic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride and 3,3',4,4'-biphenyltetracarboxylic dianhydride.
Also, the diamine compound is usually selected from 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylether, m-phenylenediamine and p-phenylenediamine.
That is, the conventional aromatic polyamic acid usually used is a condensation-polymerization product of the above-mentioned aromatic tetracarboxylic acid compound with the aromatic diamine compound. Especially, an aromatic polyamic acid prepared by the condensation-polymerization of pyromellitic acid dianhydride with an aromatic diamine is well-known and is beneficially used for various purposes.
However, the aromatic polyamic acid derived from pyromellitic dianhydride and an aromatic diamine compound exhibits disadvantageous in its solution properties and processability, as follows.
For example, when a solution of a conventional aromatic polyamic acid derived from pyromellitic dianhydride is used to coat a surface of a material and to electrically insulate the material surface, the aromatic polyamic acid solution exhibits an unsatisfactory storage stability. That is, the aromatic polyamic acid solution has a tendency of rapidly increasing its viscosity and then gelling within a short period of storage at an atmospheric temperature. When the aromatic polyamic and solution contains water, the viscosity of the solution significantly decreases within a short period of storage at atmospheric temperature. In order to prevent such rapid changes in the viscosity, it is necessary that the aromatic polyamic acid solution be stored in a refrigerator. This necessity for storage in the refrigerator causes the industrial use of the aromatic polyamic acid derived from pyromellitic dianhydride to be disadvangeous. Also, refrigeration cannot completely prevent the change in the viscosity of the aromatic polyamic acid solution if the solution is stored in a refrigerator for a long period.
Also, when another conventional type of aromatic polyamic acid derived, for example, from 3,3',4,4'-biphenyltetracarboxylic dianhydride and an aromatic diamine, is applied as an electrically insulating varnish to a surface of an electric wire, and cured thereon, the resultant cured coating exhibits an undesirable opaque appearance and has an unsatisfactory resistance to salt water (brine). Therefore, this type of aromatic polyamic acid is unsatisfactory as an electrically insulating material.
Furthermore, when still another conventional type of aromatic polyamic acid derived from benzophenonetetracarboxylic dianhydride and an aromatic diamine is used, the resultant corresponding aromatic polyimide exhibits unsatisfactory mechanical strength and heat resistance and, therefore, is not adequate for practical use.
Accordingly, a strong demand has arisen for a new type of aromatic polyamic acid solution composition which can eliminate all of the above-mentioned disadvantages of the conventional aromatic polyamic acid solution.